Yup, HDR merge in photomatix, then depending on the image, I bring it into GIMP and use different exposures of the source RAW files as layer masks to repair elements in the photos like ghosting, or chromatic aberration.

I plan on writing up a more detailed workflow for how they are created someday for anyone that's interested.

I do merge the RAW files directly in photomatix and then create the HDR and save it as 16 bit TIFF. Then bring it into GIMP with an unprocessed RAW for more fine tuning, save it as a TIFF again, then compress as a JPEG for online posting.

Here's a car I shot last night. Only one I've gotten to process thus far, there will be a few more from the night I'll post this weekend.